1. Field of the Invention
The present invention relates generally to ballasts for HID lamps, and related more particularly to a simplified topology for a ballast to drive an HID lamp.
2. Description of Related Art
High intensity discharge (HID) lamps are highly desirable for commercial markets due to their superior light efficiency in terms of lumens produced per watt and the variety of power ranges in which they are available. As HID lamp technology as developed, manufacturers and ballast designers have come to focus on reliability and efficiency in providing a robust and simple HID lighting system. For example, lamp manufacturers typically suggest that an HID lamp be driven in an AC mode, at a low frequency and with constant power to maximize reliability.
An HID lamp having a power range of 35–150 W may use less than 100 volts for ignition, which is followed by an operating region in which voltage may drop to a few tens of volts, and current may drop to a few amperes. In this operating region, it is desirable to maintain a constant power output.
When the HID lamp is ignited, a high voltage (HV) generator is placed in series with the main power supply to obtain a few kilovolts for a cold strike ignition or a few tens of kilovolts for a hot strike ignition, primarily near the point at which ignition occurs. The HV generator is fairly well known, with a number of variations presently available in conventional designs.
Lamp manufacturers suggest that lamps be driven with an AC signal before and after ignition due to the different consumption or lifetime of lamp cathode and anode terminals if a DC signal is used. Low frequency AC signals of from about 50 to about 500 Hz is suggested due to mechanical instabilities of the lamp arc upon ignition, to avoid degradation to the lamp components or eventual complete failure of the lamp.
In addition to the practical limitations illustrated by the suggestions of the lamp manufacturers, international regulations call for a limit on the harmonic current drawn by a converter connected to an HID lamp. Accordingly, to satisfy the practical considerations and the international regulations regarding operation of an HID lamp, low frequency AC signals should be used with a limitation on the harmonic current drawn by the lamp ballast or converter.
Another factor that is important for lamp operation and power converter regulation is the power factor obtained by the ballast or converter that is connected to the public power system. It is desirable that any ballast or converter for a lamp exhibit a high power factor, such as one as close as possible to unity, so that the load appears as close as possible to purely resistive as seen by the main power lines. Power factor correction circuitry draws a sinusoidal current from the main power lines that is in phase with the sinusoidal voltage supply to obtain a high power factor. A simple and inexpensive technique for power factor correction would be highly desirable.
Another important factor that contributes to longevity of an HID lamp is to regulate the power supply to the lamp during normal running conditions. Accordingly, it is desirable to provide constant power to the lamp during normal operation to maintain an even light output and extend the longevity of the lamp.
The above constraints are desirable in addition to obtaining a low frequency AC modulation for the lamp for the practical reasons discussed above.